cleanup. The work required at some sites ranges from excavation of buried waste to its
eventual disposal by incineration. Portable and mobile incinerators are being used for
wastes that do not pollute the air. Before any incineration can take place—either in
fixed or mobile incinerators—careful analysis of the effluent from the incinerator must
be made. For all these reasons, soil composition is extremely important in engineering
studies.
SPECIFIC WEIGHT OF SOIL MASS
A specimen of sand has a porosity of 35 percent, and the specific gravity of the solids is
2.70. Compute the specific weight of this soil in pounds per cubic foot (kilograms per cu-
bic meter) in the saturated and in the submerged state.
Calculation Procedure:
- Compute the weight of the mass in each state
Set V= 1 cm^3. The (apparent) weight of the mass when submerged equals the true weight
less the buoyant force of the water. Thus, Vw + V 0 = nV= 0.35 cm^3 V 5 = 0.65 cm^3. In the
saturated state, W= 2.70(0.65) + 0.35 = 2.105 g. In the submerged state, W= 2.105 - 1 =
1.105 g; or W= (2.70 - 1)0.65 = 1.105 g. - Find the weight of the soil
Multiply the foregoing values by 62.4 to find the specific weight of the soil in pounds per
cubic foot. Thus: saturated, w = 131.4 lb/ft^3 (2104.82 kg/m^3 ); submerged, w = 69.0 lb/ft^3
(1105.27 kg/m^3 ).
ANALYSIS OF QUICKSAND CONDITIONS
Soil having a void ratio of 1.05 contains particles having a specific gravity of 2.72. Com-
pute the hydraulic gradient that will produce a quicksand condition.
Calculation Procedure:
- Compute the minimum gradient causing quicksand
As water percolates through soil, the head that induces flow diminishes in the direction of
flow as a result of friction and viscous drag. The drop in head in a unit distance is termed
the hydraulic gradient. A quicksand condition exists when water that is flowing upward
has a sufficient momentum to float the soil particles.
Let / denote the hydraulic gradient in the vertical direction and ic the minimum gradi-
ent that causes quicksand. Equate the buoyant force on a soil mass to the submerged
weight of the mass to find ic. Or
ss-l
^TTe
(1)For this situation, ic = (2.72 - 1)7(1 + 1.05) = 0.84.